1. Field of the Invention
Diffractometric refractometers and a novel sample and reference cells usable in the refractometers.
2. Description of the Prior Art
U.S. Pat. No. 2,795,991 teaches an interferometer for indicating the composition of gases based on changes in refractive index including use of a comparative known reference gas.
U.S. Pat. No. 3,035,482 teaches interferometers which are employed for that type of measuring purposes in which the displacement of the interference fringes is used for determining the change in the value of a measurement, for instance, for determining the concentration of a gas.
U.S. Pat. No. 3,090,279 describes an interferometer using a diffraction grating, and either monochromatic or white light can be used.
U.S. Pat. No. 3,472,598 describes an apparatus for determining the refractive index of light permeable substances providing for measuring a reference substance of known refractive index to compare with the measurement of the unknown substance by observation of interference bands.
U.S. Pat. No. 3,487,227 describes an interferometer apparatus especially suitable for measuring gas purity and having three apertures using a monochromatic light source. Fraunhofer interference pattern is produced by this interferometer.
U.S. Pat. No. 3,612,696 describes a refractometer cell for use in liquid chromatography comprising a housing forming a chamber through which radiation can be transmitted, said chamber adapted to receive a test fluid. A radiation-transparent refracting means is positioned within the chamber. In one embodiment, this refractive means is a spherically shaped element. In another embodiment, the refractive means comprises two spaced elements having concave spherically shaped surfaces facing one another. Use with a chromatographic column in a detector is described.
U.S. Pat. No. 3,680,963 describes a refractometer to measure refractive indices of fluids by optical fringe counting. Interference fringes are created by overlapping two coherent beams of light which have traversed different optical paths. One optical path is through an unknown sample fluid and the other is through a known reference fluid. The fringe pattern shifts in direct relation to differences between the two optical path lengths.
In none of this prior art are described sample and reference cells of the design of the present invention and none of the sample cells is especially suitable because of such small sample volume for use in detectors for gas chromatographs. Thermal conductivity detectors are presently used with gas chromatographs, and the cell and device of the present invention has advantages over the thermal conductivity detectors in providing smaller internal volume sample cells (5-10 .mu.l or even 2 .mu.l or less), better sensitivity, can be used with carrier gases which may be corrosive or with corrosive samples, and is potentially simpler and lower cost.